Medical marijuana clinics in Los Angeles must shut down by Sept. 6, the City Attorney's Office is warning operators and landlords in a letter sent out this week.

The three-page letter follows a decision earlier this summer by the City Council to ban the clinics.

The letter was sent to more than 1,000 locations where the clinics are believed to be operating, and it warns they face court action and a $2,500 fine for each day they stay open past Sept. 6. The letter also went to more than 700 landlords, warning that property owners could be held liable if their tenants continue to operate dispensaries.

City officials have been meeting to develop a coordinated approach on enforcement, but nothing has been officially decided, said Councilman Jose Huizar, who authored the measure to ban the clinics.

"This is the first step," Huizar said. "We are meeting with the LAPD to discuss what steps to take next."

Kris Hermes, spokesman for Americans for Safe Access, said the letter signed by Chief Assistant City Attorney Bill Carter was puzzling because it included a detailed history of the city's efforts to control the dispensaries.

"But this was not unexpected," Hermes said of the warning letter. He added a number of other efforts are under way that could block the city's ban.

Among these is a potential referendum to overturn the Los Angeles city law, which needs fewer than 30,000 signatures to qualify for a special election.

There is a possibility the state Supreme Court could decide to hear cases involving the legality of medical marijuana clinics.

But the city's effort places the dispensary operators in an uncomfortable position, Hermes said.

"We don't have a clear-cut response for them on what's going on," Hermes said. "It would be imprudent to tell them to stay open. But at the same time, there is all this other activity going on that could change the situation in a matter of weeks."

The city does have a list of 782 dispensaries that have registered, but officials believe another 200 have also been operating.